1. Field of the Invention
The present invention relates to an electromagnetic wave shielding filter with an optical transparency.
2. Background Art
An electromagnetic wave shielding filter to be disposed on the front surface of a display has been known for shielding electromagnetic waves generated from various displays, such as PDPs (Plasma Display Panels), CRTs (Cathode Ray Tube or Braun Tube)) and the like. For the electromagnetic wave shielding filters for use in such applications, both the electromagnetic shielding ability and the optical transparency are required. To address this challenge, there has been known an electromagnetic wave shielding filter, wherein a transparent material formed of a resin film or glass and having electrically insulating properties is used as a base material, and an electrically conductive meshed layer formed by etching a metal foil or metal plating is provided on the transparent base material.
As described above, in the case of forming the meshed layer utilizing a copper foil or copper plating, since metallic luster which causes reflection of unnecessary light such as natural light degrades the contrast of perspective images in a bright room, the surface of the meshed layer is commonly covered with a blackened layer to have an appearance of a blackened face, thereby to preventing light reflection (Patent Document 1 and Patent Document 2). In this case, it is considered preferable to prescribe the surface roughness of the blackened face formed by a blackening treatment because it has a higher degree of blackness and can thus enhance the antireflection ability. For example, a reference which prescribes the surface roughness Ra in a range of 0.10 to 1.00 μm (see, Patent Document 1) and another reference prescribing the arithmetically averaged surface roughness Ra, as provided in JIS B0601, in a range of 0.02 to 1.00 μm (see, Patent Document 2) are known.    Patent Document 1: TOKUKAT No. 2000-286594, KOHO ([0022]-[0025])    Patent Document 2: TOKUKAI No. 2003-318596, KOHO ([0030])
In general, it is uncommon that the electromagnetic wave shielding filter is constructed such that the meshed layer is directly exposed to air. Namely, a transparent resin layer is provided to cover the meshed layer for protection and additional optical filter functions. Therefore, the blackened face on the side of the transparent resin layer of the meshed layer will be in the so-called wetted state with the transparent resin layer due to close lamination of the transparent resin layer thereon. Accordingly, such a blackened face will have an appearance (herein, this appearance will be referred to as “a wetted color” as is seen when a surface is wetted with a liquid) different from the appearance when it is directly exposed to air. The phenomenon exhibiting such a wetted color is the same also in the case where the blackened face is provided on the transparent base material side of the meshed layer, i.e., the case where the meshed layer having the blackened face on its transparent base material side is laminated on the transparent base material.
Accordingly, in such a usually employed configuration, the appearance of the blackened face is directly influenced by the wetted color. Therefore, in some instances, the employment of only the prescription of the arithmetically averaged surface roughness Ra as described above can not be enough for estimating the degree of blackness of the blackened face.